The present invention relates generally to cardiac pacemakers, and other types of implantable medical devices that can be programmed and/or analyzed following implantation using an external diagnostic/programmer system. Particularly, the invention relates to a high-speed digital telemetry system for use in implantable devices. More specifically, the present invention relates to an implantable telemetry transmitter and corresponding external receiver that utilize a coil driver circuitry with power savings feature for minimizing power consumption.
Implantable devices are implanted in a human or animal for the purpose of performing a desired function. This function may be purely observational or experimental in nature, such as monitoring certain body functions; or it may be therapeutic or regulatory in nature, such as providing critical electrical stimulation pulses to certain body tissue, nerves or organs for the purpose of causing a desired response. Implantable medical devices such as pacemakers, perform both observational and regulatory functions, i.e., they monitor the heart to ensure it beats at appropriate intervals; and if not, they cause an electrical stimulation pulse to be delivered to the heart in an attempt to force the heart to beat at an appropriate rate.
In order for an implantable device to perform its functions at minimum inconvenience and risk to the person or animal within whom it is used, some sort of noninvasive telemetry means must be provided that allows data and commands to be easily passed back and forth between the implantable device and an external device. Such an external device, known by a variety of names, such as a controller, programmer, or monitor, provides a convenient mechanism through which the operation of the implantable device can be controlled and monitored, and through which data sensed or detected by the implantable device can be transferred out of the implantable device to an external (non-implanted) location where it can be read, interpreted, or otherwise used in a constructive manner.
As the sophistication and complexity of implantable devices has increased in recent years, the amount of data that must be transferred between an implantable device and its accompanying external device or programmer, has dramatically increased. This, in turn, has resulted in a search for more efficient ways to effectuate such a data transfer at high speed. The telemetry must not only transfer the desired data without significant error, but it must do so at a high speed while preserving the limited power resources of the implanted device.
The problem of increasing the data transfer speed is directly affected by the power consumption of the implanted device. Typically, an implanted device has limited power capability and, due to its limited physical dimensions, allows for limited complexity of the electronic circuitry it can incorporate.
For example, a telemetry system with a data transfer speed of 8 kbps (kilobits-per-second), more exactly 8192 bps, is described in U.S. Pat. No. 4,944,299 to Silvian. It uses a carrier frequency of 8192 Hz, frequency selected to be higher than a computer monitor vertical sweep generator fundamental and its harmonics, and below the horizontal sweep generator fundamental. A computer monitor Electromagnetic Interference (EMI) is considered to be a main source of interference.
The 8 kbps telemetry system uses 1 bit per symbol and further uses a combined Amplitude Modulation (AM) and Phase Shift Keyed (PSK) modulation to limit the necessary bandwidth. The Phase Shift Keyed modulation tends to be resistant to noise because the data are encoded by means of changes in phase.
To use a carrier at 8 kbps and still increase the data transfer rate typically requires the use of more sophisticated electronics. While the design of such a system is feasible, it should be realized that this system will be implanted, in part, in an active live person. A live person has small surface movements due to respiration, heartbeats, etc. As the distance between the implant and the telemetry wand varies, so does the signal amplitude, which adds noise while demodulating finely defined amplitude levels.
An Automatic Gain Control (AGC) will minimize this effect but may not eliminate it completely. From a practical standpoint, the carrier frequency has to be increased. The following considerations have to be considered in selecting the higher carrier frequency. The implant titanium can act approximately as a 1-pole low pass filter with a xe2x88x923 dB point at 10-20 KHz. The signal above this point is attenuated as 20 dB/decade frequency, and consequently the internal power will have to be increased in the same proportion.
However, the internal power of the implanted device has to be increased more than the 20 dB/decade frequency because the carrier frequency lies within a monitor horizontal display generator and/or its harmonic. Modern monitors utilize a fundamental frequency anywhere between 16 and 75 kHz. For a 64 kbps telemetry system, a compromise is made and a carrier can be selected around 100 KHz, which is above the fundamental of the horizontal generators.
The harmonics of the horizontal generators may or may not be present at this frequency. Both, the useful 100 KHz carrier and the horizontal generator harmonics at this frequency will be attenuated by the titanium can in the range of 14 dB to 20 dB. Taking into consideration the possibility of a computer monitor interference as explained above, the transmission level at a 100 kHz carrier should be significantly higher when compared to an 8 kbps system. However, in order to render such design practical, the power source of the implanted device must be preserved.
Therefore, there is a great and still unsatisfied need for a telemetry system that allows for a high data transfer of information while minimizing the power consumption of the implanted device.
The present invention addresses these and other concerns by providing an improved telemetry system. According to a preferred embodiment, the implantable telemetry system allows a high speed transfer of digital data by minimizing the power consumption of the implanted device.
The telemetry system accomplishes this goal without increasing the complexity of the circuitry, and without significantly increasing the overall cost of the implanted device.
The foregoing and other features of the present invention are achieved by a telemetry system that communicates with an implantable device, the implantable device including a coil driver circuit to revert at least part of the expanded energy back to the power source. The implantable telemetry system further includes a timing circuit that generates two control signals SC1 and SC2.
The coil driver is comprised of four switches S1, S2, S3, S4 that are connected across the power source and the coil, and that are selectively energized by the control signals SC1 and SC2. The control signal SC1 energizes switches S1 and S4 to close, with the switches S2 and S3 open, causing the coil to become a load across the power source and to store energy therefrom.
Upon the expiration of the control signal SC1, the control signal SC2 triggers switches S2 and S3 to close with the switches S1 and S4 open, causing the power source to become a load across the coil, and the coil to discharge the stored energy through the power source charging it. In this way a large part of the coil stored energy is returned to the power source and a bypass capacitor Cbypass connected across the power source.
According to one embodiment of the present invention, the control signal SC1 is a pulse with a width T1, and the control signal SC2 is a pulse with a width T2, such that the pulse widths (or duration) of the two control signals SC1 and SC2 are related by the following equation:
T1+T2xe2x89xa6T,
where T is the duration of a single bit.
According to an alternative embodiment, the coil driver circuit includes a resonant circuit comprised of the coil and a capacitor C which is selectively connected across the coil. A switch S21 is connected between the power source and the capacitor C, and a switch S22 is connected across the coil and the power source. A diode is connected across the switch S22, for limiting the current flow to a single direction.
In this alternative embodiment, the control signal SC1 triggers the switch S21 to close with the switch S22 open, causing the capacitor to become a load across the power source and to be charged. The control signal SC2 triggers the switch S22 to close with the switch S21 open, causing the capacitor to sinusoidally discharge the capacitor C through the coil.